Devil Bringer

The Devil Bringer is Nero's right arm, and the physical manifestation of his demonic power. Although it has not been explicitly explained how Nero received the Devil Bringer, there is an Order of the Sword report that indicates that Nero received an injury to his shoulder after protecting Kyrie from a demon attack within the Mitis Forest, before the events of the game. According to Nero, "From that day forth, [his] arm changed and a voice echoed 'Power. Give me more power.'" The arm possesses superhuman strength, which it manifests telekinetically through a larger, spectral arm. It is capable of absorbing magical items, such as the Yamato or Evil Legacy, and manifesting their power. Although Nero doesn't often use his demonic arm directly, he does wield his Blue Rose with it, allowing him to channel demonic energy into the gun. The arm is constantly glowing, and the strength of the glow appears to be linked to Nero's own stamina, as it went out when Sanctus stabbed it with the Yamato and cast Nero into a coma.

Devil May Cry 4
After Nero recieved his Devil Bringer, he kept it hidden is a full-arm sleve and sling, until he first encountered Dante, when he used his arm to block a thrust from the Rebellion. He kept it hidden when around other members of the Order, but when he was alone revelaed it to better use it agains the demons that stood in his way toward Dante. After hsi encounter with Berial, the weakened demon wondered if the arm meant Nero was not human, to which Nero replied "Don't ask".

After defeating Agnus for the first time, he showed great reluctance in allowing the scientist to even look at the arm, keeping them seperated via his Red Queen. However, Agnus suprised him with a squad of Assaults and Bianco Angelos, and nearly had Nero killed in order to acquire his arm. In the moments before the Angelos attacked, Nero recalled the day he first used the Devil Bringer to save Kyrie, and the memories unleashed the Devil Trigger in him for the first time. It was then he admitted that he had accepted the consequences of Demonic power in order to keep Kyrie safe.

Unfortunately the use of the Devil Trigger caused the Order to send Nero's foster brother Credo to aprehend Nero, which ended in the latter's defeat. Unfortunately Kyrie had been witness to this and grew fearful of Nero, which allowed Agnus to capture her in order to bait the hero into following him. Nero almost rescued Kyrie once but she was taken out of his reach by Sanctus.

Following Dante's rescue of Nero, the younger devil hunter used his power to free Kyrie from The Savior and again in the final showdown with the giant statue, which he ended by crushing it's head in his spectral hand.

Throughout the game Nero displays discomfort with is arm, hiding it from other people and even admitting to Berial that "[it] drives me crazy, though]". In the end he came to accept it, telling God that he had resented the change, but now saw it as a tool to be used against evil.

Upgrade Items
The Devil Bringer is able to absorb certain Key Items and use their power to grant itself new abilities. These items are obtained either by finding them within special white pillars which project colored beams, or by taking them from specific bosses.

Evil Legacy
The Evil Legacy (負の遺産) is found within a blue pillar in the Cathedral. It is a special crystal that holds the power of an ancient evil, and after being absorbed into the Devil Bringer, it grants Nero the ability to perform Snatch by locking onto small enemies, or Hell Bound by locking onto Grim Grips and large enemies. Many Grim Grips require Nero to step upon a blue Continuum Pad to become activated, though there are more that do not. Once obtained, Snatch can be used during any mission, including those "before" it was found.

Anima Mercury
The Anima Mercury (水銀のアニマ) is found within a yellow pillar in the Library. It is an artificial soul made through alchemy that can be used to animate Gyro Blades. Nero can move a Gyro Blade by using Buster near it, and can speed it up by striking it with his sword before moving it with another Buster. Animated Gyro Blades can be used to destroy barriers and objects or shred enemies.

Rusalka Corpse
The Rusalka Corpse (ルサルカの亡骸) is one of Bael's feelers, obtained after Nero rips it off his head. After being absorbed by the Devil Bringer, it grants Nero the Auto-Search ability. When near hidden Red Orbs, Blue Orb Fragments, or Secret Missions, the Devil Bringer will begin glowing, strengthening as it gets nearer to the hidden object. Once obtained, Auto-Search can be used during any mission, including those "before" it was found; however, the glow will not be visible while charging a Charged Shot

Sephirothic Fruit
The Sephirothic Fruit (セフィロトの実) is one of Echidna's seeds, obtained after it falls from her as she flees. After being absorbed by the Devil Bringer, it grants Nero the ability to wither certain demonic trees that entangle areas of Mitis Forest and in the Order of the Sword HQ.

Aegis Shield
The Aegis Shield (アイギスの楯) is Angelo Credo's shield, obtained after Nero defeats him. After being absorbed by the Devil Bringer it grants Nero the ability to perform Hold. Once obtained, Hold can be used during any mission, including those "before" it was found.

Buster
Every enemy within Devil May Cry 4 can be damaged with Nero's Buster technique. This technique will grab the enemy and attack them in a method varying depending on the enemy. Lesser demons have different Busters depending on whether Nero is on the ground, in mid-air, or in Devil Trigger when he attacks, while Bosses can have several cinematic Buster moments which vary depending on the circumstances of both the boss and Nero. Some of these will not actually damage the boss, and will result with it using an attack on Nero. During most of these cinematic Busters, Nero can fire Summoned Swords at the boss, and for some of them he may use Exceed to increase the damage dealt.

Background
A Rusalka is a type of Slavic ghost which tries to seduce men in order to trick them into drowning.

In Greek mythology, the Aegis was Athena's shield, made from Medusa's head, and provided nearly perfect defense.

In Kabbalah, the Sephirot are the fruit of the "Tree of Life", which represents the ten aspects of God's manifestation in the material world.

Trivia

 * It is implied, but never stated, that Nero's arm is posessed by some spirit with a will of it's own, such as when Nero hears a voice demanding power.
 * The Devil Bringer's Blue color, power over the Yamato, and the power-hungry voice implies a connection with Dante's twin brother, Vergil.
 * During their fight, Dante remarks "So you too are a..." upon seeing Nero's power, and telling the novice hunter "We're the same, you and I". This has raised some debate among fans as to whether their similarities are due to Nero being a Son of Sparda or not.
 * Nero does share a number of similarities with Dante:
 * Both are orphans
 * Both have white hair and are preternaturally skilled at fighting demons
 * Both can use demonic power
 * Both have held the Yamato and Sparda.